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result of soap production

...to separate into two layers. The upper layer is neat soap, sometimes called kettle soap, of almost constant composition for a given fat (about 70 percent soap, 30 percent water); the lower, called
nigre, varies in soap content from 15 percent to 40 percent. Since colouring matter, dirt, salt, alkali, and metal soaps are soluble in
nigre but relatively insoluble in neat soap, and since most of...